A commonly-known alkaline secondary battery, such as a nickel-hydrogen secondary battery, if short-circuited due to, for example, external short-circuiting, may become heated to a high temperature for reasons of the low internal resistance and the amount of high internal energy of the battery. Accordingly, a commonly-known alkaline secondary battery includes a PTC (positive temperature coefficient) thermistor built in a positive electrode lead for electrically connecting the positive electrodes and the positive electrode terminal of an electrode group inside the battery for the purpose of ensuring safety in case of external short-circuiting (see, for example, Patent Document 1). This PTC thermistor is an electronic component composed of electrically-conductive particles. The PTC thermistor is low in electrical resistance value under normal conditions, and therefore, exhibits superior electrical conductivity. In addition, the PTC thermistor has characteristics in which the electrical resistance value of the thermistor increases drastically when the temperature thereof rises to a predetermined value.
In the alkaline secondary battery configured as described above, an excess current flows in the PTC thermistor built in the positive electrode lead if external short-circuiting occurs in the alkaline secondary battery for some reason, and therefore, the temperature of the battery rises. When the temperature of the PTC thermistor reaches a predetermined value, the electrical resistance value of the PTC thermistor increases, thereby suppressing a current flowing from the electrode group to the positive electrode terminal at the PTC thermistor-mounted portion of the positive electrode lead. The excess current flowing within the alkaline secondary battery is thus suppressed, thereby preventing the alkaline secondary battery from becoming heated if external short-circuiting occurs.
The positive electrode lead including the PTC thermistor built therein is configured by, for example, disposing two metallic strip-shaped bodies in series, partially overlapping the leading ends of the strip-shaped bodies with each other, and solder-joining the PTC thermistor between the separate and opposed leading ends of the strip-shaped bodies. The portion where the PTC thermistor is solder-joined is covered and protected with, for example, a protective member composed of alkali-resistant epoxy resin or the like. This is for the purpose of preventing the deterioration or the like of the PTC thermistor due to high-pressure oxygen generated inside the alkaline secondary battery and the deterioration or the like of soldered portions due to an alkaline ambient atmosphere inside the alkaline secondary battery.